Semiconductor Nanoparticles for Quantum Devices
by
Victor
Erokhin1,2, Sandro Carrara3,
H. Amenitch4,
S. Bernstorff4 and
Claudio Nicolini3
This is a draft paper
for a talk at the
Fifth
Foresight Conference on Molecular Nanotechnology.
The final version has been submitted
for publication in the special Conference issue of Nanotechnology.
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Abstract
Semiconductor nanoparticles were synthetized by exposing fatty
acid salt Langmuir-Blodgett films to the atmosphere of H2S.
The particle sizes were characterized by small-angle X-ray
scattering of their solutions using synchrotron radiation source
at higher resolution, as it was impossible previously to study it
with usual laboratory X-ray sources. The particle sizes were
found to correspond to the demands of single-electron and quantum
junctions. Semiconductor heterostructures were grown by self
aggregation of these particles of different types. Electrical
properties of these nanostructures were studied by using STM.
Voltage-current characteristics revealed the presence of
differential negative resistance. Measurements confirmed the
formation of semiconductor superlattices directed towards a
development of new nanodevices, like tunneling diods and
semiconductor lasers.
Introduction
The development of the electronics demands continuos decrease
of the element sizes. The aim of this trend is not only to
increase the integration level, but, mainly, to increase the
operation speed. Moreover, several specific phenomena can be
observed only when the elements have very small sizes. As one of
the examples of such phenomena we can refer to single-electron
junctions [1-3]. These interesting phenomenon takes place when a
small granule is placed between two electrodes being separated
from them by tunnelling gaps. Voltage-current characteristics of
such junction has a depression near zero bias voltage (Coulomb
blockade) or a stairway behaviour (Coulomb staircase) due to
quantization of the charge present in the granule. The phenomenon
can be observed when the electrostatic energy is higher then
thermal excitation. Therefore, the granule must be smaller than 3
nm in order to provide the possibility of observing single
electron characteristics at room temperature [4].
Even toward this aim the main stream of the development of the
electronics technology remains the same and is connected with the
continuos reduction of the element sizes by using electron beam
and X-ray lithography, dry etching processes and molecular beam
epitaxy [5-7]. Nevertheless, recently it appear several
alternative approaches, allowing to reach small sizes in a more
simple way [8-10].
In 1988 it was suggested a method of the formation of CdS
particles in Langmuir-Blodgett (LB) film of cadmium arachidate by
exposing it to the atmosphere of H2S [11]. Later the
same approach was applied to synthetize also some different
materials, such as PbS, HgS etc [12-15]. Sizes of the particles,
estimated by some not directed techniques, were found to be in
the range of 2-10 nm.
The particles were characterized also with STM and single
electron conductivity was observed realizing the structure:
conductive substrate - gap- particle - gap - STM tip [16].
Staircase characteristics with different voltage step, depending
on the particle size, were obtained. A step forward the device
application was done, when the particle was synthetized directly
on the tip of the sharp metal stylus [17]. It allowed to avoid
the localization of the particle position by STM and demanded
only to have 1D mover, for bringing the tip to the vicinity of
the conductive surface.
In parallel, it was shown the possibility to form thin
polycrystal films by self-aggregation of these particle,
performing a selective removal of the fatty acid molecules [18].
The films were characterized by different techniques. It turned
out that the average increase of the film thickness obtained from
each bilayer in a precursor is about 6 Å. Such resolution in the
thickness can be obtained only with molecular beam epitaxy.
Therefore, the suggested technique, being applied for the
heterostructure formation, can be interesting for realizing the
elements for the devices based on resonant tunnelling and light
emitting elements working without carrier recombination. The
possibility of the formation of heterostructures, containing
alternating layers of two different compounds, namely CdS and
MgS, was already shown with scanning electron microscopy [19].
Nevertheless, several questions are still open. One of them is
the statistical distribution of the particle sizes. The other
question is the properties of such prepared heterostructures. In
fact, it is not clear, whether it will be possible to obtain the
properties similar to that prepared with molecular beam epitaxy,
as in this case we are dealing with polycrystals.
The aim of this work is to characterize the statistical
distribution of the particle sizes and to realize
heterostructures by this technique.
For solving the first problem, small-angle X-ray scattering
was applied. Small-angle X-ray scattering is a well known
methodology suitable for characterizing the structure of rather
small objects in the solution. However, in the case of small
particles dispersed in a very dilute solution the intensity
provided by usual X-ray sources is not enough to obtain reliable
scattering curves and, thus, to resolve the particle structure.
Therefore, the synchrotron radiation X-ray source, providing the
intensities of several order of magnitude higher than laboratory
sources, became very important for small-angle scattering
characterization of diluted nanoparticle solutions.
Materials and methods
Arachidic acid and CdCl2 and CuSO4 5H2O
were from Sigma.
LB films were deposited with Langmuir trough (MDT, Russia)
[20]. Water used was purified with Milli-Q system till 18.2
M[Omega] cm. Monolayers were formed on the subphase, containing
10-4 M of CuSO4 or CdCl2.
Reaction of the particle formation was performed by exposing
the deposited films to the atmosphere of H2S for the
time enough to complete the reaction, depending on the film
thickness determined previously by quartz crystal balance
measurements [18].
Particle aggregation was performed by washing the films with
chloroform for selective removal of arachidic acid molecules
[18].
Particle solution was prepared by washing the aggregated film
in small amount of water. Therefore, it was impossible to
estimate precisely the concentration of the particles in the
solution.
First, the solution was investigated with small-angle
diffractometer with linear position sensitive detector (AMUR-K)
[21] equipped by a usual X-ray tube and generator (Phillips).
Second, the X-ray measurements were performed in synchrotrone
Elettra (Trieste, Italy). Elettra laboratory is a synchrotron
radiation source having more then ten beamlines dedicated to gas
phase photoemission, X-ray microscopy, diffraction and
small-angle scattering, characterisation with circularly
polarized light, VUV emission, spectroscopy, lithography (under
construction). One of this beamline is the SAXS (small-angle
X-ray scattering) beamline building up some year ago by the
Institute of Biophysics and X-ray scattering of the Graz
University under the supervision of Prof. Peter Laggner [22].
The photon source is a 57-pole wiggler, producing a 8 keV
radiation with beam size of 3.9 x 0.26 mm2 and a
maximum flux equal to 3.5 1014 photons/s/mrad (at 400
mA). The optic is constituted by a flat double mirror
monochromator and a double focusing toroidal mirror. The detector
is a one-dimensional position sensitive one (delay-line type) and
it is mounted over an movable slide in order to adjust the
sample-detector distance for the better resolution required.
Heterostructures were prepared by successive formation of
different sulphides.
The reported heterostructure was realized in the following
way. 10 bilayers of cadmium arachidate were deposited onto the
freshly cleaved graphite substrate. The film was exposed to the
atmosphere of H2S for 5 hours. Self-aggregated films
were obtained by washing the sample in chloroform. In the next
step 10 bilayers of cupper arachidate were deposited. The film
was exposed to the atmosphere of H2S for 5 hours.
Self-aggregated films were obtained by washing the sample in
chloroform. In the next step 10 bilayers of cadmium arachidate
were deposited again. The film was exposed to the atmosphere of H2S
for 5 hours. Self-aggregated films were obtained by washing the
sample in chloroform.
Voltage-current characteristics were measured through such
heterostructures, using STM for these reasons. Effectivness of
such approach for measuring lockal V/I characteristics was
already shown on epitaxially grown heterostructures [23].
Graphite substrate was used as one electrode and STM tip as the
second one, realizing the structure as shown in Fig.1.
Scheme of the realized heterostructure
STM used was one by (ASSE-Z Italy). The measuring scheme, thus,
was rather similar to that, already used for electrical
characterization of structures, grown by molecular beam epitaxy
[23].
Results and discussion
In the case of the present work, it was impossible to register
the scattering curve of the CdS nanoparticle solution (it
practically coincided with that of the solvent) with conventional
X-ray sources. Conversely, the problem was successfully solved at
Elettra synchrotron in Trieste (Italy). Scattering curves
different from that of the solvent were obtained allowing to
resolve nanoparticle sizes.
X-ray scattering curves of CdS nanoparticles in a solution of
pure water were measured in SAXS beamline. The results of these
measurements are presented in figure 2.
X-ray scattering spectrum by a diluted solution of a
monodisperse nanopartilcles
In order to obtain the nanoparticle size distribution function
it is possible to refer to the well known theory of the light
scattering by particles [24] and, in particular, to consider a
simplified version of the Debay equation to write the scattered
intensity by a system of n spherical nanoparticles
having radius R [25]:
(1)
Comparison of the theoretical scattering curve obtained from
the equation (1) and the experimental curves as one presented in
figure 2 allows to estimate the size distribution of the
nanoparticles in our solutions.
Size distribution of the nanoparticles radii
estimated by the comparison of scattering spectra as one present
in figure 1 with theoretical X-ray scattering intensity by the
simplified Debay equation (1)
Figure 3 shows such size distribution function presenting a
maxima of probability at about 30 Å. The obtained dependence
corresponds well to all results of previous indirect
characterizations. Moreover, it provides one more evidence of the
possibility of observing single electron conductivity through
such objects, as their sizes, with high probability, correspond
to that, suitable for room temperature observation.
The V/I characteristics, obtained by the described technique
over the heterostructures, had a shape shown in fig 4 a and b.
95% of all characteristics were similar to them, namely, they had
a regions with negative resistance, even if the position of the
peak was varied. The appearance of the negative resistance can be
connected to the fact, that the realized structure can be
represented as the quantum well between two tunnelling barriers
(Fig.5).
 A
 B
Different examples of V/I characteristics
obtained from the junction shown in Fig. 1.
As it was already reported, a resonant level appears in such
structures, the position of which determines the peak voltage in
I/V characteristics [26]. The position of this level (or levels)
depends upon the geometry of the junction. Therefore, it is not
strange that we have different positions of the peak for
different measurements, as our structure is formed from
polycrystal layers. Polycrystal nature of the layers results in
variation of the junction parameters, and, therefore, in position
of the peak in V/I characteristics.
Band profile of the quantum well
Conclusions
The performed measurements enabled to estimate the average
sizes distribution of the nanoparticles formed in LB precursor at
nanoscale level. The found sizes confirmed once more the
possibility of observing single-electron and quantum phenomena on
them.
V/I characteristics of heterostructures revealed the presence
of differential negative resistance, indicating the tunnelling
through the resonant levels. Not fixed position of the peaks in
V/I characteristics was attributed to the polycrystal nature of
the formed layers.
Measurements confirmed the formation of semiconductor
superlattices directed towards a development of new nanodevices,
like tunneling diods and semiconductor lasers [27].
Acknowledgments
Authors want to thank Paolo Faraci for the assistance in
providing electrical characterization with STM.
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